Engineers in general and automotive engineers in particular continuously strive for weight reduction in vehicles without compromising strength, reliability and crash worthiness. Composite materials, such as carbon fiber/epoxy composites, can provide the weight reduction and strength required by engineers. However, the connection of a composite material component to a traditional metallic automotive component can be problematic. This is particularly the case when composite material needs to be attached to traditional drive-line components, such as the engine bell housing, the transaxle, the transmission, or the differential gear unit, as non-limiting examples.
Front engine automobiles having a rear transaxle are attractive to automotive engineers because of the more equal weight distribution between the front and rear tires. Traditionally, the transaxle design uses the auto frame to hold the relative position of the engine to the transaxle and react torque loads. A drive shaft provides the power transmission between the engine and the transaxle.
A structural tube, also known as a torque tube, is typically provided between the engine and the transaxle and concentric around the drive shaft to provide a direct tie for the two components. The drive shaft runs through this structural tube. The structural tube must resist bending and torque loads between the engine and the transaxle. A carbon fiber composite tube is ideal for this application because of its lightweight, strength and dampening characteristics. A carbon fiber composite structural tube can be efficiently made by the filament winding process. The carbon fibers can be tailored in their orientation and thickness to meet the bending and torsional stiffness and strength requirements for such an application. While designing and manufacturing a carbon fiber/epoxy structural tube for the engine/transaxle torque tube application is relatively easy, the joint between the engine bell housing and the transaxle nose housing and the carbon fiber composite torque tube can be problematic.
Fasteners are often used to tie metallic and composite structural components together. However, fasteners add cost and are not attractive for high volume automotive applications. Welding components together is cost effective and typical for the auto industry but is not possible for composites. Adhesive bonded joints may be used but typically require external fixtures to hold the assemblies, such as to hold the engine/transaxle torque tube assembly, while the adhesive cures, which can be too slow for high volume automotive production.
Interference “press fit” joints are common for the automotive industry but may not be suitable for composites. If a carbon fiber/epoxy tube was “pressed fit” into or has a “press-fit” arrangement or joint with aluminum or steel bell housing for the engine/transaxle torque tube application, the composite could creep when subjected to under the hood and road temperatures. This in turn can compromise the interference fit and lead to separation. If adhesive was applied to a carbon fiber tube and it was slipped fit into the engine or transaxle housing, the adhesive would be scraped-off during installation and the strength of the bond could be compromised and highly variable.